6. COMPUTER CONTROL OF PROCESSES
Functions of a Process I/O Interface,
- Control the scanning and processing of all the inputs and outputs between the process and the computer.
- Provide conversions of signal levels, compensation for sensor range, and linearization of signals.
- Provide signal and sensor validity checks.
- Provide signal pre-processing and filtering.
- Provide measurements to the various other programming packages which have been converted to engineering units or interpreted in other ways.
- Provide alarm limit checks input-by-input.
Some features found in computer control systems,
- timer initiated events - such as clock based sampling periods for a control loop
- Process initiated interrupts - Prioritized interrupt signals can be used to alert computers to standard notifications, or to initiate an emergency sequence when a critical failure has occurred.
- Control software - directs the operating points for the control system
- System and program initiated events - these are for system house keeping, and for communication to other computer systems in a network.
- Operator initiated events - requests for reports, changing of operating parameters, checking quality, etc.
Interrupts
Each computer has the ability to process interrupts.
These interrupts can stop the microprocessor, and make it execute other subroutines.
If no interrupts are received, the computer will always run the same program
When interrupts are received, they are processed as urgent, or non-urgent.
When an interrupt is processed, a small subroutine is run, and then execution returns to the original point in the interrupted program.
If interrupts are not used, then the computer must continuously scan all of its inputs to see if any are critical. But this is not practical for real time processing.
Linear Computer Controllers,
Linear control theory can be converted into computer programs.
Canonical form (Laplace/frequency domain) control rules can be converted into a form suitable for computers by using `z-transforms'
State space control laws can be converted into computer code using matrix techniques, along with numerical integration.
For example the PID control law has been used in many computer control programs to date.
The figure below depicts a compute controlled process
Advanced topics include,
- Feed forward controllers
- Non-linear control systems
- Multi-input/Multi-output control systems
An example of Computer-process interface
